Ethernet is currently the most widespread packet-oriented layer-II protocol used in WAN data networks and in virtually every local area network (LAN).
Ethernet interfaces are defined using IEEE standards and have developed further over decades from data rates of 2 Mbit/s to 10 Gbit/s per line. Later, a method for transmitting Ethernet based data streams using synchronous SDH/SONET infrastructures was developed. This method of transmission, which describes packing of Ethernet data streams into virtual SDH/SONET containers, is standardized in ITU G.7041NY.1303 as the “generic framing procedure” (GFP). The generic framing procedure involves the Ethernet data packets being transported via a SONET/SDH network using virtual containers. These containers form permanent and reserved connection resources between network elements (SONET/SDH circuit elements).
In addition, there is a standardized method based on G.783 for forming groups of virtually concatenated SONET/SDH containers. These groups can also vary in size (=number of concatenated virtual containers). The associated method is likewise standardized in ITU G.7042 as the link capacity adjustment scheme (LCAS).
Modern SDH/SONET transport networks use an integrated control plane, which implements the rapid set-up and release of connections independently, i.e. without configuration commands from a central network management point. This involves complex end-to-connections being routed via a large number of network element entities in the network.
The first applications for transporting Ethernet data via a SONET/SDH network were limited to point-to-point connections which were set up between Ethernet client access points. In the meantime, however, network providers have been asked to provide more flexibility by allowing large numbers of client access points to interchange data with one another so that they themselves are an (Ethernet) network. This extension of the transport functionality is known as multipoint-to-multipoint Ethernet transport. For this, transport network elements now need to be extended by a plurality of layer-II protocols and by an autonomous switching function. This switching function in accordance with IEEE 802.1D allows data packets to be switched dynamically between the various client access points and/or the transport connections in the wide area network. This is referred to generally as “Ethernet bridging”.
Multipoint-to-multipoint Ethernet transport places even greater demands on monitoring the connecting capacity, however. By way of example, it is necessary to prevent data packets from being able to circulate in the network (“loops”) and hence from taking up all the available bandwidth. Such a situation may arise because packets are transported connectionlessly in an Ethernet network, i.e. incoming data packets in every network element are forwarded to the appropriate output interface in real time on the basis of the destination address they contain without knowledge of the end-to-end connection. If the destination address of a data packet is not known in the network, however, this data packet can now be forwarded from one point to the next, and thus also in loops, and may even be distributed a number of times (broadcast) without ever leaving the network. To counter this problem of circulating data packets, the spanning-tree protocol was developed for Ethernet. A further development of the spanning-tree protocol is the rapid spanning-tree protocol, which makes considerably faster reaction and convergence times available. Both protocols are standardized on the basis of IEEE 802.1d and IEEE 802.1w. These protocols ensure that a loop-free active topology is provided in any physical connection topology. This loop-free active topology corresponds to a tree structure, the so called spanning tree in which there is always precisely one clear path to a destination.
Apart from overcoming the loop problem, the spanning-tree protocol also has another function. If parts of the active topology fail, the spanning tree is recalculated and hence previously inactive resources are activated. This means that the spanning-tree protocol may also be used as an equivalent circuit protocol. These mechanisms are known in local area networks (LANs) and have also been transferred to wide area networks. As transport medium, such wide area networks use, by way of example, a connection-oriented infrastructure based on the SONET/SDH standard and also the methods described above for transmitting frame-based Ethernet data streams using this infrastructure. This gives rise to a large number of new difficulties.
The reason for these difficulties is, by way of example, that a fixed connection capacity needs to be engaged for a multipoint-to-multipoint connection in the network, and parts of this capacity are disabled by the spanning-tree protocol in order to prevent loops. Thus, the network encounters unused resources which are not used again until a resource being used fails, owing to reconfiguration of the spanning tree.
The problem is also magnified by virtue of the above unused connection capacity disabled by the spanning-tree protocol needing to be severely overdimensioned in the case of the known methods. Otherwise, large parts of the data traffic can no longer be transported by the re-adjusted spanning tree in the event of a failure without data packets being lost.
The methods known up to now have therefore required that the network be overdimensioned in order to ensure sufficient resilience. This is extraordinarily cost-intensive.